digital forensics / expert witness

I had a lawyer that is a client of mine ask me about being an expert witness. He has a client that says he sent a message on facebook and then deleted it. He thought he could use the "Download a copy" of your Facebook data feature, but I don't think that's going to work because on this page, it says:

In addition to that, I don't have any experience in criminal justice and things like "chain of custody." Can someone recommend a company that has experience in this area? This would be in the Auburn, NY area.

I'm not a forensic expert in digital forensics, anyway all starts with a proper chain of custody and an unaltered probing env, which usually means you have to perform low level dumps of stuff you intend to manage, in order to avoid to leave your traces over the previous traces (in classic HDD you want to ddrescue everything... if you can go offline).

so NO, you cannot use any account feature to demonstrate anything, unless you want your activity to be discarded (at best)